The present invention is directed to photoimageable compositions of improved composition that do not exhibit appreciable amounts of process bias and have improved resolution. The invention is also directed to methods of fabricating microstructure metal parts using the photoimageable composition and particularly to fabricating microstructures having non-linear features.
There are a variety of applications for which thick-film lithography may prove advantageous or even necessary. For instance, thick-film lithography may be used as a plating mold to create metal parts as in the LIGA process. Additionally, there may be applications in thick microelectro mechanical systems or lithographically defined analytical systems such as, for example, chromatography columns or mass spectrometers. Typically, photoresist films greater than 50-100 microns thick are exposed with a synchrotron source; the hard x-rays produced assure good transmission within the photoresist and low diffraction from the mask. In addition, the low run-out from synchrotron sources produces sharp side walls, but synchrotron sources are scarce and expensive. Moreover, long exposure times on the order of hours to days are required.
Diazonapthoquinone/novolac (DNQ/novolac) resists that are exposed with ultraviolet radiation are used in microcircuit manufacturing, however, these materials suffer from several drawbacks. DNQ produces nitrogen gas upon exposure which phase separates prior to diffusing from thick films to create bubbles. Novolac materials form highly absorbing quinones, and the DNQ direct photolysis mechanism typically results in photoresist formulations exhibiting low transmittance. Careful bake steps are required to remove the casting solvent to avoid thermally inducing reactions with the DNQ. In addition, DNQ requires water for proper formation of the soluble, photoproduced acid which leads to requiring long reabsorbption times after the initial post-apply bake (PAB). Finally, novolac materials have a tendency to crack, which is particularly problematic for thick films.
A chemically-amplified negative resist, available from MicroChem Corp., Newton, Mass., under the tradename SU-8, circumvents many of these problems. The resist includes monomers and oligomers of bisphenol A, which have been quantitatively protected with glycidyl ether, and a photoacid generator (PAG). UV exposure creates a strong acid which cationically crosslinks the oligomers during a post-exposure bake (PEB) step to form a highly crosslinked network. The resist exhibits high transmission, creates no gas during exposure, and is thermally stable. UV exposures of the resist are typically on the order of minutes, and the cured product shows excellent imaging resolution. However, drawbacks of this resist include poor solvent development, shrinkage of the cured material, and difficulty in removing the cured material.
Recently, it has been found that the SU-8 resist tends to exhibit process bias. Specifically, lines in the resist tend to be larger than the lines on the mask, while printed trenches tend to be smaller. Process bias is particularly problematic with respect to printing sharp corners and oblique shapes. There are several possible explanations. First, non-ideal aerial images, such as from diffraction can cause these effects. It has been found, however, that the process bias is relatively independent of film thickness between 100 micron and 300 microns thick, and that process bias measurements using a contact aligner are quite similar to measurements made with a projection printer. In addition, it has been found that for 100 micron thick films, the bias is identical for 80 micron features where the aerial image should be nearly ideal, as it is for 12.5 xcexcm features. For these reasons, aerial image affects appear to be only a minor contributor to the process bias. An alternate explanation is that the hexafluoroantimonic acid created upon exposure diffuses rapidly from the exposure area to cause crosslinking in the nominally unexposed areas.
The process bias depends on the radiation dose. Since it appears that increased resolution occurs with higher doses, it has been found that printing highest resolution features results in the worst process bias. The bias can be completely removed at some point by decreasing the dose, though severe loss of resolution and non-vertical side wall angles tend to result.
The invention is based in part on the demonstration that the use of selected amines in a photoimageable composition prevents process bias and improves resolution. It is believed that the amines react with the catalysts, e.g., photogenerated acids, to create an inert salt. Addition of the amines (i) reduces acid diffusion or at least the effects of small amounts of acid diffusion, and (ii) reduces the effects of small aerial image aberrations such as flare, since lightly exposed areas do not react at all. The amines may also improve the shelf-life of the photoimageable composition.
In one aspect, the invention is directed to a photoimageable composition that includes:
(a) a multifunctional polymeric epoxy resin that is dissolved in an organic solvent wherein the epoxy resin comprises oligomers of bisphenol A that are quantitatively protected by glycidyl ether and wherein the oligomers have an average functionality that ranges from about 3 to 12;
(b) a photoactive compound; and
(c) an amine that is selected from the group consisting of triisobutylamine (TIBA), 1,8-bis(dimethylamino)naphthalene (also known as PROTON SPONGET(trademark)) (PS), and 2,2xe2x80x2-diazabicyclo[2.2.2] octane (DABCO), and mixtures thereof.
Preferably, the oligomers have an average functionality of about 8 and the amine is TIBA.
In another aspect, the invention is directed to a method of fabricating microstructures that includes the steps of:
(a) forming a layer of photoimageable composition on a substrate surface wherein the photoimageable composition comprises:
(i) a multifunctional polymeric epoxy resin that is dissolved in an organic solvent wherein the epoxy resin comprises oligomers of bisphenol A that are quantitatively protected by glycidyl ether and wherein the oligomers have an average functionality that ranges from about 3 to 12;
(ii) a photoactive compound; and
(iii) an amine that is selected from the group consisting of TIBA, PS, DABCO, and mixtures thereof;
(b) exposing the layer of photoimageable composition to a pattern of radiation which produces a catalyst capable of changing the photoimageable composition""s susceptibility to a developer; and
(c) applying a developer to remove nonexposed portions of photoimageable compound which is susceptible to the developer.
In a further aspect, the invention is directed to a method of fabricating a metal structure that includes the steps of:
(a) forming a layer of photoimageable composition on a substrate surface wherein the photoimageable composition comprises:
(i) a multifunctional polymeric epoxy resin that is dissolved wherein the epoxy resin comprises oligomers of bisphenol A that are quantitatively protected by glycidyl ether and wherein the oligomers have an average functionality that ranges from about 3 to 12;
(ii) a photoactive compound; and
(iii) an amine that is selected from the group consisting of TIBA, PS, DABCO, and mixtures thereof;
(b) exposing the layer of photoimageable composition to a pattern of radiation which produces a catalyst capable of changing the photoimageable composition""s susceptibility to a developer;
(c) applying a developer to remove nonexposed portions of the photoimageable composition which are susceptible to the developer to create a mold area within exposed portions of the photoimageable composition;
(d) depositing a metal into the mold area; and
(e) removing the exposed portions of the photoimageable composition to yield the metal structure.